1. Field of the Invention
This invention relates generally to apparatus for reading and writing information patterns on a magneto-optic material and, more particularly, to a circuit for controlling the current, and hence the field, through a coil which provides the field used in conjunction with the read and write operation.
2. Description of the Related Art
The present invention relates to improvements in circuits for rapidly reversing the direction of current flow through an electrical inductor Moreover, it relates to improvements in apparatus for overwriting information previously recorded on a magneto-optic recording element.
In the magneto-optic recording process, digital information is stored in the vertically oriented magnetic domains of a magneto-optic recording element by controlling the "up" or "down" magnetization direction of each domain. The recorded information is recoverable by scan-irradiating the recorded information with a beam of plane-polarized radiation. Depending on the direction of magnetization, the plane of polarization of the reflected beam is slightly rotated, in one direction or the other typically on the order of about one or two degrees, by the well-known Kerr effect. The amount of rotation depends upon the construction and materials of the magneto-optic recording element. The combination of a polarization analyzer and a photodetector positioned in the reflected beam serves to produce an electrical signal which fluctuates in intensity in accordance with the recorded information.
In order to overwrite information previously recorded on a magneto-optic recording element, it is usually necessary to pass the recording element twice past a record/erase head. During the first pass, the previously recorded information is erased by heating the magnetic layer of the magneto-optic recording element to above its Curie temperature and then cooling the magnetic layer in the presence of the magnetic bias field normal to the recording element. This heating and cooling of the magnetic layer has the effect of uniformly aligning all magnetic domains in the direction of the bias field. In the second pass, the bias field direction is reversed, and the individual magnetic domains are selectively heated (again, to above their Curie temperature) in accordance with the digitally encoded information to be recorded. This has the effect of selectively flipping those magnetic domains which are heated, leaving unaltered the orientation of the non-heated domains.
Various schemes have been proposed to reduce the overwrite cycle to one pass in order to double the rate at which information can be overwritten. Such "direct" overwriting schemes usually require multilayered magneto-optic recording elements comprising at least two magnetic recording layers having different Curie temperatures and magnetic coerciveness. (See, for example, the disclosures of U.S. Pat. No. 4,649,519; U.S. Pat. No. 4,910,622; and U.S. Pat. No. 4,893,910). In addition to being relatively complex in construction and, hence, costly to manufacture, such multilayered recording elements require precisely controlled process parameters.
Other direct overwrite schemes have been considered in which the direction of the magnetic bias field is rapidly switched while the data track is continuously scan-irradiated by a focused continuous-wave layer which operates to successively heat each domain to above its Curie temperature. But such schemes have always been considered impractical heretofore due to the relatively slow rate at which the bias field can be switched by conventional switching circuits. (See for example the disclosure of U.S. Pat. No. 4,712,203.) To achieve data recording rates in the megahertz range, it is necessary to switch the polarity of the magnetic bias field in less than a microsecond. While it is possible using a resonant circuit to switch periodically the bias field direction at megahertz frequencies such periodic switching, without data modulation, is not useful for data recording.